c++ - 两个独立的进程共享相同的相机源 OpenCv

Question

我有两个单独的进程，它们同时使用 VideoCapture 来获取网络摄像头的图像流。有没有办法为多个进程使用相同的 VideoCapture（以有效节省资源）？

我正在考虑使用 mmap 将当前图像从一个进程传输到另一个进程，但我认为有更好的方法。有谁知道如何与 Opencv 中的两个进程共享相同的视频源？

此外，共享相同的视频捕获在计算上是否值得？还是有两个不断获取网络摄像头图像的进程在资源方面更好？

谢谢你的任何建议。

score 4 · Accepted Answer

第一个也是最好的选择是让第二个进程挂钩并拦截第一个进程的图像。这是两个进程几乎同时访问图像的最快方式。当然，一个总是在另一个之前拥有它。

如果您选择使用共享内存方式，那么以下可能对您有用：

SharedMemory.hpp：

#ifndef SHAREDMEMORY_HPP_INCLUDED
#define SHAREDMEMORY_HPP_INCLUDED

#if defined _WIN32 || defined _WIN64
    #include <windows.h>
#else
    #include <sys/types.h>
    #include <sys/mman.h>
    #include <dlfcn.h>
    #include <fcntl.h>
    #include <unistd.h>
#endif

#include <tchar.h>
#include <iostream>
#include <map>

class SharedMemory
{
    private:
        void* FromFile;
        void* hFileMap;
        void* pData;
        std::string MapName;
        std::size_t Size;
        bool Debug;
        std::map<std::string, void*> Events;

    public:
        SharedMemory(std::string MapName);
        SharedMemory(std::string MapName, std::size_t Size);
        ~SharedMemory();

        SharedMemory(const SharedMemory& Shm) = delete;
        SharedMemory(SharedMemory && Shm) = delete;
        SharedMemory& operator = (const SharedMemory& Shm) = delete;
        SharedMemory& operator = (SharedMemory && Shm) = delete;

        void* GetDataPointer();

        bool OpenMemoryMap(std::size_t Size);

        bool MapMemory(std::size_t Size);

        bool ReleaseMemory();

        bool CreateNewEvent(LPSECURITY_ATTRIBUTES lpEventAttributes, bool bManualReset, bool bInitialState, std::string EventName);

        std::uint32_t OpenSingleEvent(std::string EventName, bool InheritHandle, bool SaveHandle = false, std::uint32_t dwDesiredAccess = EVENT_ALL_ACCESS, std::uint32_t dwMilliseconds = INFINITE);

        bool SetEventSignal(std::string EventName, bool Signaled);

        bool DeleteSingleEvent(std::string EventName);

        bool DeleteAllEvents();

        void SetDebug(bool On);
};

#endif // SHAREDMEMORY_HPP_INCLUDED

SharedMemory.cpp：

#include "SharedMemory.hpp"

SharedMemory::SharedMemory(std::string MapName) : hFileMap(nullptr), pData(nullptr), MapName(MapName), Size(0), Debug(false), Events() {}
SharedMemory::SharedMemory(std::string MapName, std::size_t Size) : hFileMap(nullptr), pData(nullptr), MapName(MapName), Size(Size), Debug(false), Events() {}
SharedMemory::~SharedMemory()
{
    ReleaseMemory();
    DeleteAllEvents();
}

void* SharedMemory::GetDataPointer()
{
    void* Ptr = pData;
    return Ptr;
}

bool SharedMemory::OpenMemoryMap(std::size_t Size)
{
    this->Size = Size;

    #if defined _WIN32 || defined _WIN64
    if ((hFileMap = OpenFileMapping(FILE_MAP_ALL_ACCESS, false, MapName.c_str())) == nullptr)
    {
        if (Debug) std::cout << _T("\nCould Not Open Shared Memory Map.\n");
        return false;
    }

    if ((pData = MapViewOfFile(hFileMap, FILE_MAP_ALL_ACCESS, 0, 0, Size)) == nullptr)
    {
        if (Debug) std::cout << _T("\nCould Not Map View Of File.\n");
        CloseHandle(hFileMap);
        return false;
    }

    #else

    if ((hFileMap = open(MapName.c_str(), O_RDWR | O_CREAT, 438)) == -1)
    {
        if (Debug) std::cout << _T("\nCould Not Open Shared Memory Map.\n");
        return false;
    }

    if ((pData = mmap(nullptr, Size, PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED, hFileMap, 0)) == MAP_FAILED)
    {
        if (Debug) std::cout << _T("\nCould Not Map View Of File.\n");
        close(hFileMap);
        return false;
    }
    #endif

    if (Debug) std::cout << _T("\nInter-Process Communication Successful.\n");
    return true;
}

bool SharedMemory::MapMemory(std::size_t Size)
{
    this->Size = Size;

    #if defined _WIN32 || defined _WIN64
    if ((hFileMap = CreateFileMapping(INVALID_HANDLE_VALUE, nullptr, PAGE_READWRITE, 0, Size, MapName.c_str())) == nullptr)
    {
        if (Debug) std::cout << _T("\nCould Not Create Shared Memory Map.\n");
        return false;
    }

    if ((pData = MapViewOfFile(hFileMap, FILE_MAP_ALL_ACCESS, 0, 0, Size)) == nullptr)
    {
        if (Debug) std::cout << _T("\nCould Not Map View Of File.\n");
        CloseHandle(hFileMap);
        return false;
    }

    #else

    if ((hFileMap = open(MapName.c_str(), O_RDWR | O_CREAT, 438)) == -1)
    {
        if (Debug) std::cout << _T("\nCould Not Create Shared Memory Map.\n");
        return false;
    }

    if ((pData = mmap(nullptr, Size, PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED, hFileMap, 0)) == MAP_FAILED)
    {
        if (Debug) std::cout << _T("\nCould Not Map View Of File.\n");
        close(hFileMap);
        return false;
    }
    #endif

    if (Debug) std::cout << _T("\nMapped Shared Memory Successfully.\n");
    return true;
}

bool SharedMemory::ReleaseMemory()
{
    bool Result = false;
    #if defined _WIN32 || defined _WIN64
    if (pData)
    {
        Result = UnmapViewOfFile(pData);
        pData = nullptr;
        if (Result && Debug)
        {
            std::cout << _T("\nMemory Un-Mapped Successfully.\n");
        }
    }

    if (hFileMap)
    {
        if (CloseHandle(hFileMap))
        {
            hFileMap = nullptr;
            Result = Result && true;
            if (Debug) std::cout << _T("\nMemory Map Closed Successfully.\n");
        }
    }

    #else

    if (pData)
    {
        Result = munmap(pData, Size);
        if (!Result && Debug)
        {
            std::cout << _T("\nMemory Un-Mapped Successfully.\n");
        }
        pData = nullptr;
        return true;
    }

    if (hFileMap)
    {
        if (!close(hFileMap))
        {
            hFileMap = nullptr;
            if (Debug) std::cout << _T("\nMemory Map Closed Successfully.\n");
        }
    }
    #endif
    return Result;
}

bool SharedMemory::CreateNewEvent(LPSECURITY_ATTRIBUTES lpEventAttributes, bool bManualReset, bool bInitialState, std::string EventName)
{
    std::map<std::string, void*>::iterator it = Events.find(EventName);
    if (it != Events.end())
    {
        if (Debug)
        {
            std::cout << _T("\nCreateNewEvent Error: An Event With That Key Already Exists!\n");
        }
        return false;
    }

    Events.insert(std::pair<std::string, void*>(EventName, CreateEvent(lpEventAttributes, bManualReset, bInitialState, EventName.c_str())));
    it = Events.end();
    return ((--it)->second != nullptr);
}

std::uint32_t SharedMemory::OpenSingleEvent(std::string EventName, bool InheritHandle, bool SaveHandle, std::uint32_t dwDesiredAccess, std::uint32_t dwMilliseconds)
{
    void* hEvent = OpenEvent(dwDesiredAccess, InheritHandle, EventName.c_str());
    if (hEvent)
    {
        if (SaveHandle)
        {
            std::map<std::string, void*>::iterator it = Events.find(EventName);
            if (it != Events.end())
            {
                CloseHandle(it->second);
                it->second = hEvent;
            }
            else
                Events.insert(std::pair<std::string, void*>(EventName, hEvent));
        }
        std::uint32_t Result = WaitForSingleObject(hEvent, dwMilliseconds);
        if (!SaveHandle) CloseHandle(hEvent);
        return Result;
    }
    CloseHandle(hEvent);
    return WAIT_FAILED;
}

bool SharedMemory::SetEventSignal(std::string EventName, bool Signaled)
{
    std::map<std::string, void*>::iterator it = Events.find(EventName);
    if (it == Events.end())
    {
        if (Debug)
        {
            std::cout << _T("\nSetEventSignal Error: No Event With That Key Exists!\n");
        }
        return false;
    }
    if (Signaled) return SetEvent(it->second);
    return ResetEvent(it->second);
}

bool SharedMemory::DeleteSingleEvent(std::string EventName)
{
    std::map<std::string, void*>::iterator it = Events.find(EventName);
    if (it == Events.end()) return true;
    bool Result = CloseHandle(it->second);
    Events.erase(it);
    return Result;
}

bool SharedMemory::DeleteAllEvents()
{
    bool Result = false;
    for (std::map<std::string, void*>::iterator it = Events.begin(); it != Events.end(); ++it)
    {
        Result = Result && CloseHandle(it->second);
    }
    Events.clear();
    return Result;
}

void SharedMemory::SetDebug(bool On)
{
    Debug = On;
}

你可以像这样使用它：

第一道工序：

SharedMemory mem("OpenCVMap", 1980 * 1024 * 4); //Assuming max image size is 1980*1024*RGBA.
mem->CreateNewEvent(nullptr, true, false, "ImageReplySignal");

unsigned char* PtrToImagePixel = GetOpenCVCameraFeed();
unsigned char* MemPtr = static_cast<unsigned char*>(mem->GetDataPointer());
*reinterpret_cast<int*>(MemPtr) = GetOpenCVCameraFeedSize();
MemPtr += sizeof(int);

for (int i = 0; i < GetOpenCVCameraFeedSize(); ++i)
{
    *MemPtr += *PtrToImagePixels++;
} 
mem->SetEventSignal("ImageReplySignal", true);

第二道工序：

SharedMemory mem("OpenCVMap");
mem->OpenMemoryMap(1980 * 1024 * 4);
std::vector<unsigned char> Image;

while(true)
{
    if (mem->OpenSingleEvent("ImageReplySignal", true, true) == WAIT_OBJECT_0)
    {
        unsigned char* MemPtr = static_cast<unsigned char*>(mem->GetDataPointer());
        int size = *(reinterpret_cast<int*>(MemPtr));
        MemPtr += sizeof(int);
        Image.resize(size);

        for (int i = 0; i < size; ++i)
        {
            Image[i] = *MemPtr++;
        }

        mem->SetEventSignal("ImageReplySignal", false);
    }
}

解释：

第一个进程： 第一个进程使用“OpenCVMap”作为标识符映射一个共享内存段。它还创建一个带有标识符“ImageReplySignal”的事件，以便第二个进程可以知道何时读取。

收到图像后，它将图像大小作为整数写入共享内存区域。然后它继续将图像的内容写入内存区域。

完成写入后，它将事件设置为已发出信号。这样，第二个进程会收到一个信号，告诉它可以读取。

第二个进程： 第二个进程使用“OpenCVMap”作为标识符打开共享内存区域。在一个循环中，它不断检查是否使用标识符“ImageReplySignal”设置了信号。如果设置了事件，它会从内存区域读取大小。然后它继续从内存区域复制数据。

瞧，两个进程现在共享此图像。在第二个过程中不需要将内存区域的图像复制出来。它可以简单地在当时和那里操纵它。

在不挂钩第一个进程的情况下，这可能是让两个进程共享“完全相同”的图像/文件/视频/其他任何内容的最佳解决方案。

无论如何，在我真正提出更好的解决方案之前，最好先了解你想要什么。

c++ - 两个独立的进程共享相同的相机源 OpenCv

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